Querying Multiple Custom Taxonomy Terms

I created a new taxonomy called ‘country’ and have two terms items inside it: ‘USA’ and ‘Canada’. How do I go about querying this info? I have a filter page with two checkboxes:

<input type="checkbox" name="usa" value="USA" /> USA
<input type="checkbox" name="canada" value="Canada" /> Canada

How do I make the query show multiple results in case I have both boxes checked? I tried an array, but it didn’t work.

<?php query_posts( array('country' => array($_POST['usa'], $_POST['canada'])) ); ?>

You’ll want to use the tax_query argument for query_posts/WP_Query.

query_posts( array(
  "tax_query" => array(
    array(
      "taxonomy" => "country",
      "field" => "slug",
      "terms" => array( "usa", "canada" )
    )
  )
) );

The tax_query argument is an array, so you can query multiple taxonomies.